The present invention relates to a solution containing at least two metals and comprising at least two metal compounds.
An inorganic composite material in the form of a thin film or ultrafine particles (particles of not more than about 1 .mu.m in particle size which cannot be prepared by any mechanical grinding) in which at least two metals uniformly exist in a form of oxide or nitride has excellent electromagnetic, optical and mechanical properties and is quite useful as a functional ceramic material such as an active element.
Hitherto, various inorganic composite materials have been prepared by various processes from various starting materials. In every preparation of the composite material, it should be noted that selection of starting materials almost totally governs the characteristics of the product.
As a conventional process for preparing the functional ceramic material, there is a process in which inorganic compounds are reacted with each other in solid phase. According to the process, however, a composite material exhibiting sufficient functions and having a high purity, an ultrafine particle size and a highly stoichiometric composition cannot be obtained.
The particles of the composite material can also be prepared by dissolving metal salts of inorganic acids in water, co-precipitating the metal salts with an alkali or a polycarboxylic acid, and then thermally decomposing the co-precipitate. According to the process, however, since inorganic metal salts which form active ions in water such as a metal carbonate, a metal a chloride, a metal sulfate and a metal nitrate are employed as starting materials, there are defects in that the ions still remain in the product as impurities even if the metal salts are thermally decomposed, that the particles obtained are increased in particle size because of grain growth, and that the heat treatment should be conducted at a high temperature for preparing the desired product.
In order to eliminate the above defects, a process in which a metal alkoxide is thermally decomposed directly or, if desired, after hydrolysis treatment has been proposed. According to the process, ultrafine particles can be prepared in high purity at a lower temperature than that in the above conventional process. The metal alkoxide can be purified by chemical procedures and can provide ideal ultrafine pure particles for a functional ceramic material by thermally decomposing the metal alkoxide or heating after hydrolyzing the metal alkoxide.
According to the process, however, a functional ceramic material consisting of a single compound can only be prepared. In general, for exhibiting maximum functions of a functional ceramic material, the functional ceramic material of a single compound is seldom employed, but there is employed a material of a compound doped with the other component or a material of a composite material such as a compound oxide. Though the desired composite material can be prepared by reacting two or more kinds of the ultrafine particles of the metal oxides in solid phase with heating, the particle size of the product becomes larger during the heat treatment, and sometimes the stoicheometrics of the elements in the composite material obtained are destroyed.
Another process has been proposed for preparing a compound oxide containing two or more metals. The process comprises preparing a mixture of two or more metal alkoxides or synthesizing a composite metal alkoxide containing two or more metals, and then decomposing the mixture or the composite metal alkoxide. However, the process is not practical because of the following defects. That is, a mixture system of two or more metal alkoxides is not usually stable and cannot provide a uniform compound oxide, since metal alkoxides vary in hydrolyzing ability depending on the kinds of metal and kinds of alkoxy group. Particularly in the case that a transition metal such as nickel, copper, cobalt or zinc is employed as a metal in a metal alkoxide, almost of the metal alkoxides are insoluble in an organic solvent. Therefore a mixture system of the transition metal alkoxides in an organic solvent cannot be hydrolyzed and thus cannot provide a uniform powder. In the case of employing a composite metal alkoxide, the desired compound oxide includes impurities such as an alkali and a halogen which exert bad influences on the product, and also the yield of the product is low because in difficulty of isolating the desired product. Accordingly, when metal alkoxides are only employed, combination of the metals in the compound oxide is limited.
Another attempt has been further proposed to prepare a compound oxide by controlling hydrolyzing abilities of metal alkoxides. The control may be conducted by preparing a metal alkoxide having a large number of carbon atoms to make the rate of hydrolysis low. The attempt is not also practical, however, since the control is complicated due to the variety of the properties of the metal alkoxide. Also the stability of the mixture of the metal alkoxides is not improved.
A metal alkoxide has been used for forming a metal oxide thin film other than for preparing ultrafine particles. The process avails hydrolysis of a metal alkoxide. A metal oxide thin film can be easily formed by applying a metal alkoxide to a substrate, hydrolyzing with moisture in air, and thermally decomposing. The process is practically adapted to a film of tetraethyl silicate for an electrically insulating film, a passivation film for alkali migration or anti-reflection film; a film of tetraisopropyl titanate for a heat reflective film, an optical filter or an undercoating for accelerating adhesion to an organic compound. However, from the above reasons there is no practical technique for preparing a compound oxide thin film from metal alkoxides.
A technique for preparing a thin film of an inorganic compound is an important fundamental technique in the electronic industry. As the technique, there is mainly employed a process utilizing a vacuum technique such as a vacuum deposition method or a sputtering method. Wide availability of a thin film of a compound oxide is expected in an optical device, a memory, a piezoelectric element, a sensor, and the like, and the thin film is partially practised. However, the adaptation of the vacuum technology for the thin film is limited because of an expensive equipment cost, a high utility, difficulty of preparation of a large film, difficulty of preparation of a film made of an oxide having a high melting point, and a low productivity. Further, it is difficult to prepare the desired product having good stoichiometry and crystallinity, because the vapor pressures of the metals are different and some metals having various valencies produce various compound oxides.
On the other hand, the process for preparing a metal oxide using a metal alkoxide has advantages in that a simple and nonexpensive equipment can be employed, that a large film can be prepared, that a cheap product can provided because of high productivity, and that a uniform film having an excellent stoichiometric property can be prepared. Therefore, if a process in which various compound oxides can be prepared by using metal alkoxides is practised, it is clear that various new uses and new functions of the compound oxides will be found.
For instance, a novel multilayer film having various functions can be prepared by laminating two or more compound oxide films.
Another adaptation is expected as a functional ceramic material in the electronic devices such as a thermistor, a varistor, a resistor, a capacitor, a filter, a magnetic core and a sensor. Such devices exhibit their functions by utilizing the specific phenomena in the interface and on the surfaces of the ceramic particles as well as the inherent properties of the functional ceramics. For instance, there is a varistor which is produced by placing bismuth oxide in the interface of electrically conductive zinc oxide particles, and a boundary layer capacitor having a high dielectric constant prepared by diffusing an acceptor ion such as bismuth ion, copper ion or manganese iron in the interfaces of a semiconductive ceramic device made of barium titanate, titanium oxide, strontium titanate, and the like. In order to prepare such electronic elements, it is important that the ceramic particles have an even particle size and that the boundary layer is thin and uniform as much as possible. Those conditions are also required to produce a compact and thin electronic element used in an integrated circuit which operates under a low voltage and a low energy. However, the above electronic devices and elements are rarely produced or are produced by complicated processes.
Furthermore, there has been proposed an attempt in which a novel functional element is prepared by uniformly dispersing ultrafine particles having functions in itself into the other ceramic material. For instance, if ultrafine magnetic particles are uniformly dispersed in a supporting material, an excellent magnetic recording material having a high density may be provided. Also there is a possibility that the functional element provides a sensor having a new function.
However, according to the conventional processes such as a powder molding process, it is very difficult to produce a ceramic material which includes two or more metals because the ultrafine particles undergo secondary agglomeration due to their large surface energy. Accordingly, even if the ultrafine particles of a metal oxide are prepared by thermally decomposing a metal alkoxide, it is hard to disperse the ultrafine particles uniformly in the other ceramic material.